The present invention relates to an apparatus for applying coating material on a moving web of paper, and in particular to coating apparatus of the trailing blade type wherein accurate control over flow of material to and pressure of material in an application zone, and treatment of the material to homogenize the same, is accomplished in novel and improved manners.
Conventional coaters of the trailing blade type include means for applying coating material to a paper web that is usually supported and carried by a resilient backing roll, together with a flexible coater or doctor blade located on the trailing side of the applicator, which serves to doctor or level the applied coating. In general, an excess of coating material is applied on the web and the coater blade then meters or removes the excess while uniformly spreading the coating onto the web surface.
In recent years it has become desirable to produce papers having a minimum amount of coating. To achieve low coat weights with conventional trailing blade equipment it is necessary to increase the pressure of the coater blade against the web, which results in a high rate of wear of the blade the necessitates more frequent blade replacement. High blade pressure also increases the possibility of web breaks and streaking caused by foreign particles caught between the blade and web.
Conventional coaters employ a relatively long dwell or soak time, which is the time interval between initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as some of the water soluble or dispersible materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive web coats without intervening drying, because the successive coats tend to migrate into and contaminate the previous coat.
To overcome the disadvantages of prior applicators and to apply lightweight coatings on paper, there has been developed a short dwell time applicator as disclosed in U.S. Pat. No. 4,250,211, issued to Damrau et al. and assigned to the assignee of the present invention. In that applicator, coating material is introduced in excess into a relatively narrow application zone for being applied on a web of paper carried therethrough. A forward wall of the applicator defines a relatively narrow gap with the web at the upstream end of the zone, and excess material in the zone overflows through the gap and forms therein a liquid seal, so that coating material in the zone and as applied to the web is maintained under pressure. The speed of the web is adjusted for a relatively short dwell time, and a flexible coater blade doctors the web at the downstream end of the zone, thereby removing excess material from the web and at the same time uniformly spreading the material on the web. In consequence of the short dwell time of the pressurized application of coating material on the web, an appropriate yet lightweight amount of coating may be applied without need for high blade pressures.
The coating material which flows through the gap between the forward wall and the web to form the liquid seal is returned to the supply for recirculation, which prevents stagnation and hardening of the material in the application zone. However, despite recirculation and although new coating material is added to the supply to replace that applied on the web, with the passage of time the viscosity of the material changes and interferes with application of a uniform coating on the web.
In addition to the viscosity of the coating material affecting the uniformity of coating applied on the web, also of importance to the quality and quantity of coating is the pressure of the material within the application zone. The pressure in the zone is influenced by the pressure of the material within the chamber, along with the size of the metering slot between the chamber and zone and of the orifice between the front wall and the web. Therefore, the sizes of the metering slot and orifice are adjustable and the pressure of coating material introduced into the chamber is controllable to control the pressure in the zone. However, while the sizes of the orifice and metering slot are not ordinarily subject to to change once adjusted, it often happens that changes occur in the pressure of coating material introduced into the chamber, and slight variations in the pressure cause variations in the pressure in the application zone and affect the coating rate, either by causing too much or too little coating material to be applied on the paper.